The invention relates to a glow discharge source (GD) for the elemental analysis of solid samples by means of optical glow discharge spectroscopy (GD-OES) or glow discharge mass spectroscopy (GD-MS) or secondary neutral particle mass spectroscopy (SNMS). The inventive glow discharge source may be operated with direct current or with pulsed direct current or with HF voltage.
In the case of the known glow discharge sources, aglow discharge is produced by means of a connected electrical voltage source on the sample of material between the latter and an anode and this is evaluated spectrometrically (EP 0 636:877; DE 41 00 980; V. Hoffmann; H.-J. Uhlemann; F. Prxc3xa4xcex2ler; K. Wetzig; Fresenius J. Anal. Chem. (1996) 355: 826-830).
For the glow discharge sources, operated with direct current (DC-GD), the current usually is measured in the voltage source. When glow discharge sources (RF-GD), operated with a high frequency voltage, the power, running to the source or to the adapting network and reflected by the source, and the high-frequency current and the high-frequency voltage are measured.
The known current measurement has the disadvantage that a large idle current Tbl=Tblan+Tblka is superimposed on the plasma current Tpl. Since water-cooling furthermore is integrated in the known glow discharge sources, a portion of the current Twiwa flows to ground because of the finite conductivity of the water. As a result, the current, leaking away over the water cooling, is also disadvantageously detected by the measuring equipment so that the plasma current Tpl, which alone is relevant for the spectroscopy, is distorted.
It is therefore an object of the invention to configure a glow discharge source for the elemental analysis of solid samples, for which a glow discharge is produced between the sample of material and an anode by means of a connected electrical voltage source, in such a manner, that the idle current Tbl is minimized and the current Twiwa, flowing over the cooling water, is not measured.
Pursuant to the invention, this objective is accomplished with the glow discharge source, which is described in the claims.
Pursuant to the invention, a current transformer component for detecting the current flowing between the glow discharge and the current source, is disposed at or in the anode or the components connected electrically with the anode.
Pursuant to appropriate embodiments of the invention, the current transformer component may be a coil or a Hall generator. The current transformer component may also be an ohmic resistance, which is inserted in the connecting piece of the anode and connected with an ammeter.
In the event that an HF voltage source is used, the current transformer component advantageous is surrounded by HF shielding.
Compared to known sources, the inventive glow discharge source is distinguished by the fact that the current measurement is integrated in the source, since the current, flowing in the region of the anode, which is grounded at the generator, is converted into a measurement signal. By these means, only the current, supplied to the glow discharge or the plasma, is measured and conditions, reproducible for different samples of material, are ensured. With that, the quality of the spectrometric results is improved significantly.
The invention is explained in greater detail below by means of examples and the associated drawings.